A male donkey can indeed be bred to a mare. The resulting offspring of such a pair is a sterile hybrid animal called a mule.
Horses and donkeys are separate species, but the two can interbreed. The result of a female donkey mated with a stallion is called a hinny, while the result of a male donkey mating with a mare is called a mule. Mules are more common than hinnies, because they are easier to breed.
However, breeding a donkey with a mare can be difficult if the two animals are not familiar with one another's species. Male donkeys are often uninterested in mating with mares, unless they have been raised among horses. Mares that are not familiar with donkeys can be fearful and refuse to respond to mating attempts by the male. Male donkeys can also be very aggressive towards mares in heat, causing injury.